This invention relates to improved itaconate copolymers useful as contact lens materials.
Early hard contact lenses were produced from polymethyl methacrylate (PMMA) or cellulose acetate butyrate (CAB). Later, rigid, gas permeable (RGP) contact lenses formed of silicone-containing copolymers were introduced, offering various advantages over PMMA and CAB lenses, particularly increased oxygen permeability. The original silicone-containing RGP lenses were based on copolymers of a silicone-containing monomer and methyl methacrylate, and such lenses remain on the market today.
A newer class of copolymers for silicone-containing RGP lenses is itaconate ester copolymers. U.S. Pat. Nos. 4,152,508 (Ellis et al.), 4,330,383 (Ellis et al.) and 4,826,889 (Ellis et al.) disclose copolymers for contact lenses prepared from: a monofunctional siloxanyl ester monomer; an itaconate ester; an ester of a monohydric or polyhydric alkanol or phenol and a (meth)acrylic acid; a crosslinking agent; and preferably a hydrophilic monomer.
Known copolymers for RGP lenses also include copolymers of fluorinated itaconate esters, such as the copolymers disclosed in U.S. Pat. Nos. 4,686,267 (Ellis et al.) and 4,996,275 (Ellis et al.), which are prepared from a fluorinated itaconate ester and an ethylenically unsaturated organosiloxane.
Other examples of itaconate ester copolymers for RGP lenses are disclosed in the following U.S. Pat. Nos. 4,602,074 (Mizutani et al.); 4,508,884 (Wittmann et al.); 4,743,667 (Mizutani et al.); 4,826,936 (Ellis); and 4,861,850 (Novicky).
As disclosed in U.S. Pat. No. 4,152,508, the itaconate ester provides rigidity, hardness and some degree of wettability to the resultant copolymer. However, the inclusion of an itaconate ester tends to make the resultant copolymers more brittle.
Certain multifunctional organosiloxanes have been described as useful for adding higher impact strength and reducing brittleness of itaconate RGP copolymers. U.S. Pat. No. 4,826,936 describes a class of multifunctional organosiloxanes having the formula: ##STR2## wherein n is 0 to 10, the total "a" values is at least 2, each Y' is an unsaturated polymerizable group, and the remaining variables have the meanings given in the patent. A preferred monomer for itaconate copolymers is 1,3-bis(methacryloxypropyl)-1,1,3,3-tetra(trimethylsiloxy) disiloxane (designated SM-6). Additionally, the aforementioned patents, U.S. Pat. Nos. 4,686,267 and 4,996,275, disclose that the fluorinated itaconate copolymers may include a multifunctional organosiloxane of U.S. Pat. No. 4,826,936. U.S. Patent No. 4,743,667 also discloses multifunctional organosiloxane monomers for RGP contact lens materials described as having high impact strength and reduced brittleness of itaconate ester copolymers. Preferred monomers include 1,5-bis(methacryloxypropyl)-1,1,3,3,5,5-hexamethyl trisiloxane (designated BiMAPPS-1) and 1,3-bis(methacryloxyethoxypropyl)-1,1,3,3-tetramethyl disiloxane (designated BiMAPPS-2). The approaches suggested in each of these patents involve employing a relatively rigid multifunctional organosiloxane wherein the number of siloxane units bridging the polymerizable functional groups is preferably 2 to 4.